1. Field of the Invention
The present invention relates to a flying height tester and a flying height test method which determines the flying height optically with the phenomenon of multiple reflections produced between a glass disc and a magnetic head slider.
2. Description of the Related Art
The magnetic head (read/write head), which is an element of a magnetic read/write system, flies at a small air gap from the, surface of the spinning magnetic recording disc. The flying height of a magnetic head is usually tested using a flying height tester (FHT) in a form of a head gimbal assembly (HGA), according to the specifications thereof.
Various types of flying height testers have been developed. A particular type of flying height tester which determines the flying height optically with the phenomenon of multiple reflections has been widely used.
Such a particular type of flying height tester is provided with a transparent glass disc as a substitute for a magnetic disc. The flying height is determined with this type of flying height tester in the following procedure.
Firstly, the glass disc is driven to spin in a state where a light beam emitted from a light source is guided perpendicularly on an upper surface of the glass disc. Subsequently, a magnetic head slider is brought to a state in which the slider faces the spinning glass disc so that the magnetic head slider flies over a spinning disc while maintaining a small air gap against the disc surface. Thereupon, the illuminated light from the upper surface of the glass disc is reflected more than once between a lower surface of the glass disc and an upper surface of the magnetic head slider, as shown in FIG. 2, due to the differences in refractive index among the glass disc, the magnetic head slider and the air in the gap between the glass disc and the magnetic head slider. Subsequently, light rays reflected by the lower surface of the glass disc and the upper surface of the magnetic head slider are received by a light sensor (photosensor) to determine the intensity of the reflected light, and the flying height is derived from this determined intensity.
The reflectivity R of the lower surface of the glass disc is expressed as a function of the flying height (flying height vector) x as shown by the following theoretical formulas (1), (2) and (3):
                              R          ⁡                      (                          x              ->                        )                          =                                            R              12                        +                          R              23                        -                          2              ⁢                              r                12                            ⁢                              r                23                            ⁢                              cos                ⁡                                  (                                                            α                      ⁢                                                                                          ⁢                                              x                        ->                                                              -                    ϕ                                    )                                                                          1            +                                          R                12                            ⁢                              R                23                                      +                          2              ⁢                              r                12                            ⁢                              r                23                            ⁢                              cos                ⁡                                  (                                                            α                      ⁢                                                                                          ⁢                                              x                        ->                                                              -                    ϕ                                    )                                                                                        (        1        )                                                      r            12                    =                                                    n                1                            -              1                                                      n                1                            -              1                                      ,                              R            12                    =                      r            12            2                          ,                              r            23                    =                                                                                          (                                          1                      -                                              n                        3                                                              )                                    2                                +                                  k                  3                  2                                                                                                  (                                          1                      -                                              n                        3                                                              )                                    2                                +                                  k                  3                  2                                                                    ,                              R            23                    =                      r            23            2                                              (        2        )                                          α          =                                    4              ⁢              π                        λ                          ,                  ϕ          =                                                    tan                                  -                  1                                            ⁡                              [                                                      k                    3                                                        1                    -                                          n                      3                                                                      ]                                      +                                          tan                                  -                  1                                            ⁡                              [                                                      k                    3                                                        1                    +                                          n                      3                                                                      ]                                                                        (        3        )                            wherein the refractive index of the air in the gap between the glass disc and the magnetic head slider is defined as 1;        wherein R represents the reflectivity of the lower surface of the glass disc;        n1 represents the refractive index of the glass disc;        n3 represents the refractive index (optical constant) of the magnetic head slider;        k3 represents the extinction coefficient (optical constant) of the magnetic head slider; and        λ represents the wavelength of the reflected light.        
If the reflectivity R(x) of a lower surface of the glass disc which is positioned directly above the magnetic head slider is known, the flying height x which corresponds to the reflectivity R(x) can be derived from the above theoretical formulas (1), (2) and (3). However, the reflectivity R and the flying height x are calculated back from light intensity data I based on the following theoretical formula (4) since data obtained by the light sensor is not the reflectivity R but the light intensity data I:{right arrow over (I)}=GR({right arrow over (X)})+offset   (4)
In the above theoretical formula (4), the intensity gain data G and the intensity offset data OFFSET need to be calibrated prior to the measurement of the flying height.
Conventionally, the intensity of the reflected light (light intensity data) is obtained for each of different flying states of the magnetic head slider. Thereafter, the intensity gain data G and the intensity offset data OFFSET are calibrated using a peak value (maximum value) Imax and a valley value (minimum value) Imin of the obtained light intensity data I in accordance with function data (i.e., output function of the light sensor; see FIG. 11). The function data is previously derived from the distance between the glass disc and the magnetic head slider and the light intensity data, for each wavelength region in accordance with an optical theory. Such a conventional calibration method is disclosed in the following documents: Japanese patent application Laid-open No. 6-147841, Japanese patent application Laid-open No. 7-65331, Japanese patent application Laid-open No. 7-503315 and Japanese patent application Laid-open No. 8-507384.
As described above, it has been previously necessary to obtain the light intensity data I, which includes both the peak value (maximum value) Imax and the valley value (minimum value) Imin, in order to calibrate the intensity gain data G and the intensity offset data OFFSET. For instance, assuming that the light emitted from a light source consists of three primary colors: RGB (red, green and blue), the range of the flying height which is necessary for obtaining light-intensity data I′ (IR′, IG′ and IB′ at wavelengths R, G and B, respectively) for calibration becomes maximum at wavelength R as shown in FIG. 9. Specifically, the magnetic head slider needs to be lifted by approximately the order of 300 nm. If only the light-intensity data IB′ at wavelength B is intended to be used, the magnetic head slider needs to be lifted by approximately 220 through 230 nm.
In recent years, the surface recording density of magnetic discs has been increased remarkably, and the flying height is becoming smaller as the surface recording density of magnetic disc increases. The flying height is, at present, approximately 20 nm, and it is anticipated that the flying height will be reduced to 10 nm or less in the future. In addition, current magnetic head sliders are designed to fly relative to the magnetic disc while remaining parallel to the magnetic disc (with a small pitch between magnetic head slider and magnetic disc) so as to ensure a stable flying position. Therefore, it is difficult to secure the maximum flying height value necessary for obtaining the light intensity data I′ for calibration even if the magnetic head flies under different load conditions as well as a single load condition. On this account, it is sometimes the case that the valley value Imin of the light intensity data I cannot be obtained. If the valley value Imin is not obtained, the accuracy of calibration of the intensity gain data G and the intensity offset data OFFSET deteriorates, and in the worst scenario, the intensity gain data G and the intensity offset data OFFSET cannot be calibrated.
It is known that the light intensity data I, in a state where the flying height is nearly zero (10 nm or less), greatly depends on the refractive index n3 and the extinction coefficient k3 of the magnetic head slider. The refractive index n3 and the extinction coefficient k3 of the magnetic head slider are values measured by an external measuring device such as an ellipsometer. Therefore, if measurement errors occur in refractive index n3 and the extinction coefficient k3, such measurement errors have a great influence on the determination of the absolute flying height to thereby cause an offset error in the flying height.